1. Technology Field
The present invention relates to a carbon-dioxide-capture-type steam power generation system for capturing carbon dioxide from exhaust gas produced when steam is produced by combusting fuel in a boiler, by using an absorbing solution for absorbing the carbon dioxide, while generating electric power. In particular, this invention relates to the carbon-dioxide-capture-type steam power generation system which can restrain the power generation efficiency from lowering and can take out energy required for capturing the carbon dioxide from the exhaust gas.
2. Background Art
Recently, in the field of thermal power generation equipment, intensive study for enhancing power generation efficiency has been made in view of environmental preservation. For instance, in usual thermal power generation equipment, the temperature of the steam used for operation of a steam turbine is 600° C. or so. However, the recent study has been aimed at enhancing the power generation efficiency by further elevating the steam temperature.
As methods for enhancing the power generation efficiency, a coal-gasification-combined-cycle power generation using coal which is a fossil fuel that is of great reserves and available at a relatively low price, and a steam power generation utilizing supercritical pressure steam produced by a pulverized coal boiler are known. Among them, the coal-gasification power generation can enhance the power generation efficiency by elevating temperature of the gas supplied to a gas turbine. Meanwhile, the steam power generation can enhance the power generation efficiency by driving the steam turbine, while elevating the temperature and pressure of the steam (or turbine steam) produced by the dust coal boiler and supplied to the steam turbine. In fact, in this steam power generation, it has been attempted to enhance the power generation efficiency by elevating the temperature of the turbine steam up to 700° C. or higher, presenting some expectations of commercialization in the near future.
Incidentally, in recent years, the so-called greenhouse effect due to carbon dioxide has been pointed out as a major cause of global warming. Thus, there is an urgent need to restrain from emitting the carbon dioxide, in order to preserve the earth's environment. To this end, one method for capturing the carbon dioxide contained in the exhaust gas, by contacting the exhaust gas generated in a thermal power plant or the like, with a carbon dioxide absorbing solution, has been intensively studied.
Generally, a carbon dioxide capturing system includes an absorbing unit being configured to absorb the carbon dioxide contained in the exhaust gas into the absorbing solution, and a regenerating unit being configured to release a carbon dioxide gas from the absorbing solution absorbing the carbon dioxide and discharge the released carbon dioxide gas containing the steam (see JP2004-323339A, JP2005-254212A, and JP2006-342032A). To the regenerating unit, a compressor compressing the carbon dioxide gas containing the steam which is discharged from the regenerating unit, and a reboiler producing the steam (or regenerating steam) by heating the absorbing solution which is supplied from the regenerating unit, are connected, respectively.
In the case of capturing the carbon dioxide by utilizing such a carbon dioxide capturing system, the exhaust gas generated by the boiler is supplied to a lower portion of the absorbing unit, while the absorbing solution is supplied to an upper portion of the absorbing unit. Thus, the exhaust gas containing the carbon dioxide and the absorbing solution are brought into countercurrent contact with each other, thereby the carbon dioxide contained in the exhaust gas is absorbed in the absorbing solution. The exhaust gas, from which the carbon dioxide has been taken out or removed, is discharged from the upper portion of the absorbing unit.
The absorbing solution absorbing the carbon dioxide is supplied to an upper portion of the regenerating unit from the absorbing unit, via a lean/rich heat exchanger. Meanwhile, high temperature regenerating steam, which is produced by heating the absorbing solution in the reboiler, is supplied to a lower portion of the regenerating unit. Thus, the absorbing solution absorbing the carbon dioxide and the regenerating steam are brought into countercurrent contact with each other, thereby the carbon dioxide gas is released from the absorbing solution absorbing the carbon dioxide. The carbon dioxide gas containing the steam is discharged from the regenerating unit and then compressed by the compressor. The compressed carbon dioxide gas is supplied and stored in desired equipment. In this way, the carbon dioxide gas contained in the exhaust gas can be captured. As a usual heat source for heating the absorbing solution to produce the steam in the reboiler, the turbine steam produced in the boiler and extracted therefrom is used.
The absorbing solution used for absorbing the carbon dioxide comprises an aqueous amine compound solution prepared by dissolving an amine compound (or compounds) in water. The following amine compounds are mentioned.
For instance, JP61-71819A discloses acidic gas scrapping compositions containing a non-aqueous solvent, such as a steric hindrance amines, a sulfolane, or the like. Among the steric hindrance amines, 2-amino-2-methyl-1-propanol or the like is mentioned as a steric hindrance primary mono-amino-alcohol. JP5-301023A discloses use of particular aqueous hindered amine solutions exemplified by hindered amines selected from the group consisting of 2-amino-2-methyl-1-propanol, 2-(methylamino)-ethanol, 2-(ethylamino)-ethanol, 2-(diethylamino)-ethanol, and 2-(hydroxyethyl)-piperidine. Alternatively, JP8-252430A discloses use of a mixed aqueous amine solution containing a secondary amine and a tertiary amine, respectively, at concentrations within a range of 10 to 45% by weight. Further, “Chemical Engineering Science” (Vol. 41, No. 4, pp. 997-1003) discloses behavior of an aqueous solution of 2-amino-2-2-methyl-1-propanol, one kind of the hindered amines, upon absorbing the carbon dioxide.
While such various kinds of absorbing solutions that can absorb the carbon dioxide have been reported, in any case of using the absorbing solution containing such an amine compound as described above, there is a need for directly contacting the absorbing solution with the exhaust gas containing the carbon dioxide, in order to capture the carbon dioxide from the exhaust gas.
Generally, the aqueous amine compound solution is prepared, as the absorbing solution, by dissolving the aforementioned amine compound or compounds in water. Such an absorbing solution causes an exothermic reaction when the carbon dioxide contained in the exhaust gas is absorbed into the absorbing solution, while this absorbing solution causes an endothermic reaction when the carbon dioxide is released from the absorbing solution absorbing the carbon dioxide. Therefore in the case of releasing the carbon dioxide from the absorbing solution, it is necessary to heat the absorbing solution. This is the reason that the high temperature regenerating steam produced by the reboiler is supplied to the regenerating unit and the absorbing solution absorbing the carbon dioxide is heated by the supplied regenerating steam in the regenerating unit. Incidentally, the concentration of the carbon dioxide that can be contained in the absorbing solution is increased, as the temperature of the absorbing solution is decreased. Therefore, in order to allow the carbon dioxide to be effectively absorbed in the absorbing solution, there is a need for positively cooling the absorbing solution so as to decrease the temperature of the absorbing solution.
As described above, in the case of capturing the carbon dioxide contained in the exhaust gas by using the aqueous amine compound solution, as the absorbing solution, some amount of heat is required for heating the absorbing solution absorbing the carbon dioxide. Specifically, the amount of heat, within a range of approximately 2.5 to 3.5 Mj/kg.CO2, is required. This amount of heat corresponds to approximately 10 to 20% of the heating value of the coal. Thus, when it is attempted to take out energy, corresponding to such a necessary amount of heat from the turbine steam that is originally used for rotating the steam turbine, the energy required for rotating the steam turbine is decreased by the amount. This leads to lowering of the power generation efficiency. However, in the case of capturing the carbon dioxide without utilizing the aqueous amine compound solution, it has been also reported that the power generation efficiency may be further lowered. Generally, in such a case, it has been known that the power generation efficiency may be lowered by about 30%.
In addition, in order to elevate the temperature of the turbine steam up to about 700° C., several challenges to be overcome still remain. For instance, if the temperature of the turbine steam supplied to the steam turbine is considerably elevated, the degree of superheating the turbine steam in a steam inlet of the steam turbine, especially a low pressure steam turbine, tend to be markedly increased. Thus, even if the degree of vacuum is substantially elevated in a steam outlet of the low pressure steam turbine, it becomes quite difficult that the turbine steam can reach a saturated temperature at the steam outlet of the low pressure steam turbine. In such a case, it also is difficult to keep the turbine steam in an adequately wet state. Therefore, a dried state and a wet state are alternately repeated (or a dry-wet cyclic state is generated), in a final stage blade of the low pressure steam turbine, leading to serious corrosion of the final stage blade.
In the thermal power plant, a feed-water heater is provided for heating, in advance, feed water which will be supplied to the boiler by using the turbine steam extracted from the steam turbine as heating source. As described above, when the temperature of the turbine steam is elevated up to, for example, approximately 700° C., the temperature of turbine steam (or extraction steam) extracted from the steam turbine and used as heat source for the feed-water heater is also elevated. Usually, however, for heating the feed water in the feed-water heater, latent heat of condensation of the extraction steam is mostly utilized, in order to keep the interior of the feed-water heater at substantially the same temperature and prevent the feed-water heater from deforming. Therefore, in the case of considerably elevating the temperature of the turbine steam, the degree of superheating the extraction steam tend to be excessively increased, causing the feed-water heater to be unduly deformed.